This invention relates to a process and apparatus for the production or treatment of flat glass in an elongate tank containing a bath of molten material. The process includes delivering glass to the bath and advancing the glass along a path in contact with the bath towards an outlet for the glass from the tank. The invention has particular, but not exclusive, reference to the manufacture of flat glass by the well known "float process".
During the production of glass by the float process, it is usual for molten glass to be fed onto one end of the bath in the tank where it is allowed to spread out. As the glass advances along the tank, irregularities in its thickness and surface are removed and it is allowed to cool and passed to the tank outlet where it is fed onto conveyors passing it to an annealing lehr and a cutting station.
These thickness and surface irregularities may be considered as being exclusively due to the way in which the molten glass spreads out on the bath to form the ribbon, and they are substantially removed as the ribbon advances along the tank, mainly in regions where the glass has low viscosity.
There is, however, a further source of faults in a ribbon or sheet of glass as it advances along a bath of molten material. A sheet or ribbon of glass is prone to faults arising from this cause as it advances along the tank in contact with the molten material in the bath. These faults are apparent as surface defects in the glass and are not readily removed by its further passage along the tank. Such a surface defect will typically occupy an area of the ribbon which is in the form of a spindle the central part of which is one or two centimeters wide and which extends along the moving path of the ribbon for a distance of, at most, 40-50 cm.
These surface defects are generally detected by well known testing methods as, for example, the Zebra-test which is described in particular, in ATZ "Automobiletechnische Zeitschrift," 57, No 11, November 1955, pages 335-338.
As the glass advances along its path in the tank, molten materia in the surface layers of the bath is entrained and also flows along the path. The molten material flows along the path slowly than the glass advances, and it is believed that this difference in speed is at least partly responsible for the formation of these surface defects since it has been found that the greater the speed difference, the more pronounced they are. The occurrence of such surface defects at a given region also depends on the viscosity of the glass at that region.